Vista - Can see XP computer from Vista but can't access it

We have 2 XP PCs and 2 Vista Home. The Vista laptops can see everybody on the network map and can connect to each other and their printers, but they can't connect to the XP computers. The XP machines can't see anybody else (although they used to before we installed a new router). All can get to the Internet just fine.

This is what I have done:
1. I made sure all PCs are in the same workgroup, the network is private, and I set up the same user names on each PC.
2. Network Discovery, etc., all except password protected sharing is On.
3. Firewall: Made sure that the IP range was allowed, and when that didn't work, disabled the firewall. (Tried McAfee, then Windows - tried them both, one at a time, then none - i never had them both running at the same time). Also, set them up to allow ping requests and opened system services ports as recommended by Microsoft in the Vista File and Printer Sharing document.
4. Made sure that all of the sharing was set up properly on the other PCs and printers.
5. Changed the default WPA2 security on the Linksys router to WPA, then to WEP, then to no security.
6. Reset router. Upgraded firmware.
7. AP Isolation is not checked on the router, unchecked Filter Anonymous Internet Requests and associated setting.

I did most of these things to fix the problem we had with one Vista PC not able to see anyone, but that was resolved. We still have the problem with the XP machines.

Oh yeah, installed the patch.

Thanks.

6/11/09: Fixed the problem. It isn't enough to mess with the firewall settings. With McAfee, you have to use their EasyNetwork to share files and printers. You have to go through a setup and access approval process, a little time-consuming, before you can do anything. So there you have it.

Thanks to all of you who at least took a look at my question, even if you didn't have an idea.